Orbital arc welding equipment is often utilized to weld tubing and piping used in fluid handling and processing systems. For example, U.S. Pat. No. 5,136,134 to Benway et al. (herein incorporated by reference), and U.S. Pat. No. 4,223,686 to Benway et al. (herein incorporated by reference) both disclose orbital welding apparatuses for welding a pair of tubular workpieces. The apparatuses disclosed therein are primarily directed to systems for welding small diameter tubing in closely confined spaces. The '134 and '686 patents disclose a tube clamping assembly for receiving and retaining the tubes to be welded during welding operations. The tube clamping assembly disclosed therein is a generally scissors-type clamp, wherein each tube is held in place between a movable clamp arm and a stationary clamp arm. A tube is mounted in each pair of arms such that they are in end-to-end abutting relation. Once the tubes have been so secured, the orbital weld head is inserted into the tube clamping assembly to carry out the welding operations, and the tubes are welded at their abutting joint. The present invention is directed to improvements in the tube clamping assembly disclosed therein and in other similar tube clamping assemblies.
When welding cylindrical workpieces, the workpieces must be radially and axially aligned such that the end of the first workpiece abuts the end of the second workpiece. However, manufacturing tolerances limit the precision of radial alignment that can be attained in most tube clamping assemblies. Accordingly, there exists a need for a tube clamping assembly capable of ensuring precise radial alignment between the tubular workpieces. An additional problem often arising with existing scissors-type tube clamping assemblies is that the fixed clamp arms, which are typically made integral with the base, are not removable. Accordingly, when the fixed clamp arms are damaged, the entire fixture may have to be replaced. Thus there is a need for a tube clamping device having removable clamp arms, particularly removable stationary clamp arms.
In order to ensure proper alignment of the tubes to be welded, axial movement of the clamp arms should be minimized. It has been found that the clamp arms may flare axially outwardly during clamping of the cylindrical workpieces. The reaction of the radial clamping forces upon the outside diameter of the tube may transfer into an axial force component which causes the clamp arms to flare outwardly. The outward movement may shift the tubes apart so that their butt ends are no longer in contact. Furthermore, it has been found that shrinkage of the weld joint during welding may cause inward movement of the clamp arms. The inward movement may cause the arms to pinch or clamp upon the weld held by the clamping assembly, which can impair the weld head rotor rotation. Accordingly, there is a need for an arrangement for limiting the axial movement of the clamp arms in both the inward and outward axial directions.
Finally, each set of clamp arms should clamp the workpiece with sufficient force such so as to securely grip the workpiece therein to avoid any shifting during welding operations. For example, U.S. Pat. No. 4,868,367 to Benway et al., hereby incorporated by reference, discloses one such apparatus for gripping and retaining a workpiece within a collet. However, existing collets may not provide sufficient contact points between the collet and the tube, thereby allowing the tube to shift with the collet. Accordingly, there is a need for a tube clamping assembly having a collet which ensures adequate points of contact between the collet and the tube.